Automated variable pitch propeller blade

This application is a continuation of U.S. patent application Ser. No. 17/749,915, filed May 20, 2022, which is a continuation of U.S. patent application Ser. No. 16/844,672, filed Apr. 9, 2020, which claims priority to U.S. patent application Ser. No. 15/231,783, filed Aug. 9, 2016, the contents of which are incorporated by reference in its entirety.

This disclosure relates to propeller blade designs, and more specifically, to a propeller blade system that changes the blade pitch in different flight conditions.

Aerial vehicles such as quadcopters are reliant on the propeller blades to liftoff, hover, and directionally fly. Fixed pitch propeller blades are only designed to be maximally efficient at one particular flight condition. Therefore, the efficiency of the fixed pitch propeller suffers during significant portions of flight. Propeller blades that are able to vary the blade pitch are conventionally controlled through mechanical systems that require the input of a pilot. Instead of being efficient at only one flight condition, the propeller may be controlled to be increasingly efficient during many different conditions. However, these mechanical systems are prone to inaccuracies, mechanical failure, and/or human error. There appears to be lacking mechanisms to accurately adjust the blade pitch during flight without human intervention.

A propeller blade includes a first material and a second material. The first material includes fibers. The second material is different from the first material. The fibers are interspersed through the second material and the fibers are oriented in a same direction within the second material. The propeller blade is anisotropic and includes sections of the fibers.

A propeller blade including multiple sections, a first material, and a second material. The first material includes fibers. The second material is different from the first material. The fibers of the first material are interspersed through the second material in a direction perpendicular to a chord line of the propeller blade. A volume fraction of the first material varies in the multiple sections.

A propeller blade including a first material and a second material. The first material includes fibers that extend along a radius of the propeller blade. The second material different from the first material. The fibers of the first material are interspersed through the second material. The propeller blade is twisted along a length of the blade to generate different blade pitches.

The figures and the following description relate to preferred embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of what is claimed.

Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the disclosed system (or method) for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein.

Overview Configuration

Disclosed by way of example embodiments is a variable pitch propeller blade that alters the blade pitch depending on the flight condition (e.g. takeoff versus hover conditions). In various embodiments, the propeller blade is designed with sections that comprise materials with different stiffness levels, hereafter referred to as the elastic modulus of the section. As the propeller blade increases in rotational velocity, the sections respond to the generated lift forces by changing their blade pitch to minimize the drag forces exerted on the propeller blade. In turn, this maximizes the efficiency of the propeller blade. In varying embodiments, the sections may be designed with different materials and structural configurations to achieve a propeller blade that varies the blade pitch depending on the flight condition.

A propeller blade with a variable pitch propeller may maximize a lift during takeoff and minimize drag forces at hovering conditions. As an aerial vehicle, for ease of discussion herein referenced as a quadcopter, lifts off, each cross-section of the propeller blade, hereafter referred to as an airfoil cross-section, may be oriented with a blade pitch that enables the generation of a significant amount of lift along the radius of the blade. When the propeller blade is at higher rotational velocities (e.g. flying or hovering conditions), airfoil cross-sections may reduce their blade pitch, thereby minimizing the surface area exposed to the incoming wind. This minimizes drag forces while maintaining flight speed. In addition to improving the efficiency of the propeller blade, the variable blade pitch may be achieved in-flight without the need for human input, thereby minimizing errors, e.g., human or mechanical, that often accompanies conventional mechanical control systems.

Example Propeller Blade

Referring now to, it illustrates a propellerviewed from a top-down perspective, in accordance with an example embodiment. In one embodiment, the propellercomprises two opposing propeller blades (or blades)coupled via a connector. The rootof the bladeis nearest to the connector, whereas the tipof the bladeis farthest from the connector. The distance between the rootand the tipis hereafter referred to as the radius of the blade. Even thoughdepicts two separate blades, in some embodiments, the propeller may include three, four, or more bladescoupled together via the connector. It is noted that although the structural elements of the propellerhave been individually identified, the propellermay be either comprised of one of more of the elements fit together, e.g., via adhesives and/or mechanical connectors, or may be a unibody construction.

The airfoil, which is a cross-section at a particular point along the blade, may have significantly different designs depending on its location along the radius of the blade. For example, an airfoil at the rootof the blademay have a significantly different composition than an airfoil at the tip of the blade. In some embodiments, the bladeis designed with a particular twist along the length of the blade. The blade twist is the change in blade pitch proceeding along the radius of the blade from the rootto the tip. Given that lift increases exponentially with the rotational velocity of the blade, the tipof the bladeexperiences significantly higher quantities of lift as compared to the rootof the blade, especially at higher rotational velocities. Therefore, the blade twist may be designed to provide proportionate amounts of lift across the radius of the blade. In some embodiments, the rootof the blademay have the highest blade pitch whereas the tipof the bladepossesses the lowest blade pitch.

Varying the Pitch of the Propeller Blade

Turning now to, it illustrates an airfoil cross-section at the tipof a blade, during low propeller speeds, in accordance with an example embodiment. The example embodiment shows designations for the trailing edgeand leading edgeof the blade, a chord lineof the blade, a relative wind direction, a vector extending from the imaginary horizon, an angle of attack (), and a blade pitch (). The angle of attackmay be referenced as the angle between the chord line, which connects the trailing edgeto the leading edge, and the relative winddirection. The blade pitchmay be referenced as the angle between the chord lineand the imaginary horizon vector.

For ease of discussion relative to the figure, the relative windmay be assumed to be approaching in a directly horizontal manner, thereby paralleling the imaginary horizon vector. Decreasing the blade pitch from ϕ 225 to ϕ′will result in a corresponding decrease in the angle of attack from θ 220 to θ′.

As currently illustrated in, the airfoil may have an angle of attackof θ. In some embodiments, the angle of attackmay be between 1 and 20 degrees. To maximize generated lift at takeoff conditions, the airfoil cross-section at the blade tip may hold the angle of attackat 20 degrees. However, at higher propeller speeds, a small angle of attack (e.g. angle of attackof 1-2 degrees) may be preferred to minimize the drag forces.

illustrates an airfoil cross-section at the tipof a bladeat high propeller speeds, in accordance with an example embodiment. In various embodiments, the reduced blade pitch ϕ′may be achieved by displacing only the trailing edgeupwards while holding the leading edgein place. In other embodiments, the reduced blade pitch ϕ′may be achieved by shifting the leading edgedownward while holding the trailing edgein place.

Althoughdepict a particular example design of the airfoil, one skilled in the art may envision a variety of different airfoil shapes that can achieve the same changes in blade pitch by displacing the trailing edgeor the leading edge. This includes varying intrinsic parameters of the airfoil including the camber, maximum camber length, thickness, maximum thickness, and chord length.

Furthermore, one skilled in the art will understand that althoughdepict the airfoil cross-section at the blade tip, the subsequent embodiments may depict airfoil cross-sections at different points along the radius of the blade. Airfoils at different locations along the radius of the blade may differ substantially in the intrinsic airfoil parameters (e.g. camber, thickness, chord length). However, structural embodiments for achieving a particular elastic modulus may be applied to airfoil cross-sections at different locations of the propeller blade.

Elastic Moduli of a Variable Pitch Propeller

In one embodiment, a model propeller may have a blade radius of 3 inches. During take-off, the blade tiphas a pitch atof 20 degrees and changes to a blade pitch ϕ′of 1 degree at hovering conditions. By way of example of a quadcopter, there are 4 propellers, and each propeller has 2 blades, each propeller blade must support ⅛ of the total quadcopter weight of 15 pounds (6.8 kg). At the blade tip, the chord length, C, is approximately 0.2 inches (5 mm).

Modeling the airfoil as a cantilever beam that is fixed on the leading edge, the elastic modulus, E, of the airfoil may be calculated using the beam equation:

where F is the uniformly applied lift force on the propeller blade, C is the chord length, Δd is the desired displacement of the trailing edge, and I is the moment of inertia of the propeller blade, e.g., blade. The desired displacement of the trailing edgemay be calculated by the equation

where C is the chord length and ϕ and ϕ′ are the blade pitch at takeoff and hovering speeds, respectively. Therefore, the estimated elastic modulus, E, of the trailing edgeof the blade tip is approximately 8 GPa in order to achieve a blade pitch change from 20 degrees at takeoff to 1 degree at hovering conditions. To ensure that limited deflection occurs for the leading edge, the elastic modulus of the leading edgemay be several-fold higher (e.g., 100 Gigapascal (GPa)).

One skilled in the art will be able to readily appreciate that the calculations provided here are for an airfoil located at the blade tipwith a particular set of characteristics. Altering the airfoil parameters such as the chord length will have an effect on the desired elastic modulus of the airfoil.

Laminar Structure of an Airfoil

illustrates an airfoil design of a bladewith a modular structure, in accordance with an example embodiment. In one example embodiment, the modular structure comprises two separate sections. Each section may be composed of a different material with different structural properties. Additionally, the first sectionincludes the leading edgewhereas a second sectionincludes the trailing edge.

In various example embodiments, the first sectioncomprises a material that has a large elastic modulus whereas the second sectioncomprises a material with a low elastic modulus. For example, given that the desired elastic modulus of the trailing edgewas previously calculated to be 8 GPa, the second sectionmay be composed of a material that has the desired elastic modulus of 8 GPa. Therefore, at hovering speeds, the second sectionsubstantially bends, as illustrated inand the trailing edgeis displaced upward to achieve the desired reduction in the blade pitch to ϕ′. Alternatively, the first sectionmay be composed of a material that is at least several-fold higher above a threshold value. For example, the first sectionmay have an elastic modulus that is >100 GPa, at least 10 times higher than the elastic modulus of the second section. This ensures that the first section withstands the lift forces and maintains its structural orientation at hovering speeds.

In some embodiments, the elastic modulus of the second sectionfalls within an appropriate range of elastic moduli. For example, at takeoff, the pitch of the propeller blade may be oriented at 10 degrees instead of 20 degrees to reduce the risk of stalling. At hovering conditions, the pitch of the blade may be reduced to 3 degrees to maintain a minimum amount of generated lift. With these new parameters, the desired elastic modulus of the second section is calculated to be 22 GPa. Therefore, the appropriate range of the elastic modulus of the second sectionwould be 8-22 GPa.

In, the airfoil is not limited by the depicted orientation of the firstand secondsections. Althoughdepicts an airfoil with only two sections of two different materials, other example embodiments may include three or more (e.g. tens, hundreds, or more) different sections. In some example embodiments, the different sections may be organized as an individual layer to form an airfoil with a laminar structure. In some example embodiments, the individual layers comprising the laminar structure may be anisotropic. This example embodiment is further discussed below with.

In some example embodiments, the first sectioncomprises a first material such as carbon fiber whereas the second sectioncomprises a second material such as reinforced polycarbonate. Other types of materials that may be chosen include, but are not limited to, reinforced plastics (e.g., fiber reinforced plastics, glass reinforced plastics), fiber glass, aramid fibers, polypropylene, aluminum, polyacrylonitrile, pitch, and rayon. In some embodiments, a single section may comprise two or more materials as a composite material to achieve the desired elastic modulus of 20-40 GPa or >100 GPa.

Interspersion of Different Materials in an Airfoil

illustrate an airfoil of a bladewith one material interspersed throughout a second material, in accordance with an example embodiment. In one example embodiment, the structure of the airfoil is anisotropic and includes individual fiber sections composed of a first materialwith a high elastic modulus. These individual fiber sections may be oriented perpendicular to the plane of the airfoil cross-section and travel longitudinally between the blade rootand blade tip. The fiber sections of the first materialare interspersed in a matrix section of the second materialthat possesses a smaller elastic modulus. The configuration illustrated inmay be referred to as a “longitudinal fiber structure.” To achieve the desired elastic moduli at the leadingand trailingedges, the volume fraction of the firstand secondmaterials may be tailored.

For example, the leading edgeof the propeller blade may contain a high volume fraction of the first materialand a low volume fraction of the second materialto achieve a higher elastic modulus. Conversely, in the trailing edge, sections of the first materialare sparsely scattered throughout a higher volume fraction of sections of a second materialto obtain a lower elastic modulus.

Given that the lift forces generated by the rotating propeller blade act upwards, the force vector is perpendicular to the individual fibers of the first material. Therefore, in this embodiment, the transverse elastic modulus, E, of the composite material is considered. Using the rule of mixtures for the transverse elastic modulus, Eis calculated by

where Eand Eare the elastic moduli for the first and second material, respectively and Vis the volume fraction of the first material.

As previously determined, the desired transverse modulus for the trailing edge would be approximately E=8 GPa. In one embodiment, the first materialmay be composed of individual carbon fiber with an approximate elastic modulus of 150 GPa. The second materialmay be polycarbonate with an approximate elastic modulus of 3 GPa. Therefore, for a transverse modulus of E=8 GPa, carbon fibers amount to 65% of the volume fraction of the trailing edge whereas polycarbonate would account for 35% of the volume fraction. In the case of the leading edge, the desired modulus may be significantly higher at E=100 GPa. Therefore, 99% of the airfoil volume fraction is carbon fiber whereas 1% is polycarbonate. In other embodiments, the first materialmay be glass fiber (E=180 GPa) and a second materialmay be nylon (E=2 GPa).

In one example embodiment, proceeding from the leading edgeto the trailing edgeof the airfoil of the blade, the volume fraction of the first materialdecreases in a continuous fashion. For example, as described in the previous embodiment, the volume fraction of carbon fiber may decrease linearly from 99% at the leading edge to 65% at the trailing edge. In some embodiments, the decrease occurs and may be modeled by an exponential decay, a logarithmic decay, or a polynomial decay model.

In some example embodiments the volume fraction of the first materialdecreases in a non-continuous fashion when proceeding from the leading edgeto the trailing edge. For example, the blademay have discrete sections with a constant volume fraction of the first materialinterspersed through a second material. As depicted in, the volume fraction of the first materialabruptly drops through a vertical delineation between the leading edgeand trailing edge.

The example embodiments depicted inare not meant to be limiting. For example, the fibers of a first materialmay be directionally oriented to travel along the chord lineof the airfoil cross-section as is illustrated in. This configuration may be referred to as a “ribbed structure.” Similar to, the volume fraction of the first materialat the training edgeis significantly lower than the volume fraction of the first materialat the leading edge.

Additionally, although, the first materialis shown to be a uniformly circular fiber that is traveling longitudinally through the airfoil cross-section of the blade, the interspersed fiber of the first materialmay be of a different shape (e.g. square, rectangular) and the fiber diameter may be larger or smaller than depicted.

depicts the fibers of a first materialoriented vertically (e.g. vertical fibers) in the airfoil. This configuration also may be referred to as a “ribbed structure.” Vertical fibers may be referred to as fibers that travel in the plane of the airfoil and are perpendicular (or are ±10 degrees of being perpendicular) to the chord lineof the airfoil of the blade. Therefore, the vertical force vector of the generated lift on the airfoil cross-section may be applied longitudinally along the fibers. The rule of mixtures is similarly used to calculate the composite longitudinal elastic modulus, Eas

Returning to the prior example involving carbon fiber (150 GPa) as a first materialand polycarbonate (3 GPa) as a second material, the trailing edgeis composed of 4% carbon fiber and 96% polycarbonate to achieve a composite elastic modulus E=8 GPa. At the leading edge, 66% carbon fiber and 34% polycarbonate yields a composite elastic modulus of E=−100 GPa. The manner in which the volume fraction of the first materialdecreases from the leading edgeto the trailing edgemay also occur following a linear, exponential, logarithmic, or polynomial fit.

One skilled in the art may recognize that the design of the airfoil is not limited to a first materialand a second material. Additional materials may be included as fibers interspersed in a matrix material. For fibers that are oriented perpendicular to the lift forces, the composite elastic modulus may be calculated from the equation

where V+V+ . . . +V=1 and E, E, . . . Eare the respective elastic moduli for each material. Similarly, for fibers that are oriented longitudinally with the applied force, the composite elastic modulus may be calculated from the equation

In some embodiments, the vertically oriented fibers of the first materialmay be organized into an anisotropic laminar structure as described in the prior embodiment in. For example, the first layermay be a composite material that includes, by volume, 66% of vertically oriented carbon fibers and 34% polycarbonate matrix for an elastic modulus of 100 GPa. The second layermay be a composite material that is, by volume, 4% vertically oriented carbon fibers and 96% polycarbonate matrix for an elastic modulus of 8 GPa. Because the rule of mixtures is only concerned with the volume percent of each material, in both the first and second sections, the individual fibers may be randomly interspersed in the second materialas long as they are vertically oriented.